1. Field of Invention
The present invention relates to a light emitting device including a light emitting element with high current efficiency of light emission, and having high display quality.
2. Description of the Related Art
In recent years, development of a light emitting device or a display device using a light emitting element which contains an organic material or an inorganic material, has been actively carried out. A light emitting element is manufactured by interposing an organic compound or an inorganic compound between a pair of electrodes. Differing from a liquid crystal display device, since a light emitting device using alight emitting element emits light by itself, the light emitting device requires no light sources such as a backlight. In addition, the light emitting element itself is very thin. Therefore, the light emitting device has great advantage to manufacture a thin and lightweight display.
A light emitting element has an organic material or an inorganic material between a pair of electrodes. By applying current to the light emitting element, a light emitting material is excited and so on so as to obtain a predetermined emission color. It has been known that luminance of light emission of such a light emitting element increases in proportion to the amount of current. However, it has been also known that applying the large amount of current promotes deterioration of the light emitting element. That is, although the luminance of light emission can be increased by feeding the large amount of current to the light emitting element, this promotes deterioration of the light emitting element. If high luminance can be obtained with a smaller amount of current, lifetime of the light emitting element can be prolonged.
Accordingly, it is proposed that high light-emission luminance can be obtained by laminating a plurality of light emitting elements, and by feeding current with the same amount of current density as the case of one light emitting element (see patent document 1). In accordance with the patent document 1, predetermined luminance can be obtained in the laminated light emitting elements even when current with a half the amount of current density required for one light emitting layer is fed. For example, in order to obtain n times luminance at a desired amount of current density, by providing n pieces of light emitting units each having the same structure between electrodes, n times luminance can be realized without increasing the current density in accordance with the patent document 1. In this case, the driving voltage is also increased n times or more. It is described in the patent document 1 that there is a great advantage of being capable of realizing n times luminance without shortening the lifetime.
In the case of the structure as described in the patent document 1, however, an equipotential surface is provided using indium tin oxide (ITO) between a light emitting unit and another light emitting unit. When the equipotential surface is formed across a plurality of pixels, so-called cross talk is caused by an adverse influence of potential of adjacent pixels.
Further, since the equipotential surface has a conducting property, when the equipotential surface is exposed from a light emitting unit and in contact with an electrode, initial failure due to short-circuiting is caused.
Furthermore, when the equipotential surface is formed to correspond to a light emitting region, slight misalignment is caused between the equipotential surface and the formation position of the light emitting region. Therefore, it is thought that difference in brightness might be caused within the light emitting region or a misaligned position between the equipotential surface and the light emitting region might become a non-light emitting region. Accordingly, the slight misalignment adversely affects display quality significantly.
[Patent Document 1]: Japanese Patent Application Laid-Open No. 2003-45676